Field of the Invention
The invention relates to a method for determining the injection time for a direct-injection internal combustion engine wherein a high-pressure reservoir is provided from which fuel is delivered to an injector, and an injection time for the injector is ascertained in dependence on a value for a required injection mass of fuel and of the pressure in the high-pressure reservoir.
Internal combustion engines with direct injection are very promising with regard to reducing fuel consumption with relatively low pollutant emissions. In contrast to intake pipe injection, in direct injection fuel is injected at high pressure directly into the combustion chamber.
Injection systems with a central pressure reservoir (common rail) are known in the prior art. In those so-called common rail systems, a fuel pressure regulated by an electronic control unit of the engine, via pressure sensors and pressure regulators, is built up in the common rail and is available largely independently of the engine speed (rpm) and the injection quantity. The fuel is injected into the combustion chamber via an electrically driven injector. The injector receives its signals from the control unit.
By functionally separating the pressure generation and the injection, the injection pressure can be selected largely freely, regardless of the current operating point of the engine.
Modern electronically controlled injection systems for diesel engines are time-controlled, as compared with the old mechanically controlled systems. In the latter, the reference point is the compressed volume and they moreover have a fixed relationship between the feed rate and the engine speed, and hence a fixed relationship between the injection pressure and the speed. This means that in conventional systems, with the same adjustment of the quantity adjusting mechanism, the same fuel volume is always injected regardless of the ambient conditions prevailing at the time. By comparison, in time- controlled systems, a triggering time must be ascertained on the basis of a calculated injection mass.
In common rail systems, in which the injection pressure can accordingly be adjusted independently of the operating point of the engine, the injector triggering time required for a particular injection mass is dependent, under constant ambient conditions, on the pressure drop prevailing just at that time at the injector, i.e. the injection nozzle.
In the control unit, the requisite triggering time is ascertained from a characteristic map, which is plotted for the required injection mass and the current pressure in the pressure reservoir (common rail). The basis for the map are measurements made on a test bench, in which the rail pressure and the triggering time are varied, and the injection masses resulting from the variation are measured. The injectors then inject counter to the ambient pressure. When the data obtained in this way are entered into a memory of the electronic control unit, injection events that are incorrect in terms of the injection mass can occur during engine operation, as a consequence of prevailing pressure conditions at the injection nozzle that differ from those on the test bench.